The present invention relates to a method and a device for preventing thermal damage of workpieces during grinding.
During the manufacture of high quality components such as toothed wheels and other workpieces with special profiles, it is of great importance, for providing high precision and trueness to form, after premanufacturing and after the hardening process to provide a further fine machining. As a further machining step grinding is commonly used. In this grinding step, a predetermined amount of material must be removed which corresponds to excess material resulting from premanufacturing before the hardening, to deformations caused by the hardening or to inaccurate positioning of the workpiece on the grinding machine. In the case of toothed wheels, the amount of material to be removed is usually within a range of 1/10 to 3/10 of a millimeter.
The final shape of the workpiece is achieved by grinding off the excess material whereby in some cases tolerances of only a few micrometers are allowable, especially with toothed components and other profiled workpieces in the aviation industry. On the other hand, due to economic and cost considerations, the fine machining should be performed in a short period of time in order to maximize the production rate per machine and per unit of time.
These two contrary requirements, on the one hand high precision and on the other hand removal of a substantial material excess within a short period of time have resulted in the development of highly efficient grinding processes, wherein, besides the conventionally employed grinding materials such as silicone carbide, electrocorundum etc., diamonds and cubic-crystalline boronitride (CBN) are also being used.
In the industrial practice, the fine machining process is carried out at high rates of volume of grit per time unit, that is, with a maximum number of removed cubic millimeter of material per second of grinding time and per millimeter of grinding disk width. Especially, when cubic-crystalline boronitride (CBN) is used high numerical values for the respective volume of grit per time unit may be achieved. Reference values for a maximum volume of grit per time unit and thus for the optimization of the grinding process with respect to economic considerations may be taken from common industrial practice.
The limit for the optimization of the respective process, however, is met where grinding is carried out under such high advancement and feeding rates that the grinding disk at the contact surface of the workpiece reaches locally high temperatures which result in thermally caused structural changes of the workpiece surface layers. Such a thermal damage to the workpieces will result in a high discard rate.
For the industrial production of toothed wheels it is therefore important to provide a stable grinding process that will not cause thermal damage. In practice, in the manufacture of toothed wheels, especially in man production, it has been shown that there are no finishing process which at a high economic efficiency will ensure that no thermal damage occurs. It is therefore common monitoring practice to use so-called nital etching processes wherein workpieces removed at random from the production line are etched in different baths in order to make visible structural changes of the surface portions of the workpiece. However, this process is relatively expensive and time consuming and does not provide a sufficiently reliable screening of the machined workpieces. These etching processes are also very costly and time-consuming. Furthermore, these processes are only usable in a subsequent testing in order to separate damaged pieces from quality pieces. At the time of the testing the finishing process has already taken place and the damage finished workpieces to be discarded. Today's practice is thus damage recognition but not damage prevention.
Tests have shown that thermal damage occurs when during the grinding process the amount of material to be removed per time unit has been set to an excessive value. Then, per second of grinding time and per millimeter of grinding disk width too many cubic millimeters of material are removed. The respective value of the amount of material per time unit may also be calculated by multiplying the area to be subjected to grinding with the advance of the grinding tool relative to the workpiece.
Today there are highly efficient grinding tools available on the market with which it is possible to achieve a defined value for the respective amount of material to be removed per time unit for a given time period without creating thermal damage. According to the aforementioned definition of the respective amount of material per time unit, two parameters determine a flawless finishing process without thermal damage: on the one hand, the rate of advancement of the grinding tool and, on the other hand, the effective amount of material to be removed which depends on the premachining of the tool, on deformations during the hardening process and/or the positioning of the tool during grinding. These conditions are at least valid for full-form grinding processes in which with one or with only a few grinding steps the entire amount of material is removed. This grinding process is commonly used in the mass production of toothed wheels nowadays. In principal, these conditions hold true for any kind of hard fine machining respectively finishing.
In practice, it has been proven that an economical and technically flawless grinding process may be achieved when workpieces are machined with high quality tools and optimized rates of advancement within the grinding machine while the amount of material to be removed will be kept within a given tolerance value. The problems concerning thermal damage however, may occur when unpredictably great increases of the amount of material to be removed occur so that the grinding tool is extensively loaded. These deviations of the amount of material to be removed may occur when a different lot of tools is to be ground in which the manufacturing has been inexact and/or increased deformations due to the hardening process have occurred.
It is therefore an object of the present invention to provide a method to prevent thermal damage of workpieces, i.e. thermal loads of a workpiece during grinding. The method should be simple and, in critical cases, should provide for a change of the grinding step conditions before the grinding step occurs so that high quality workpieces may be manufactured without discard.